Alternating Current (AC) universal electric motors are used in a wide variety of applications involving power tools such as drills, saws, sanding and grinding devices, yard tools such as edgers and trimmers, just to name a few. These devices all make use of electric motors having an armature and a field, such as a stator. The armature is typically formed from a lamination stack or core around which a plurality of windings of magnet wires are wound. The lamination stack is formed to have a plurality of poles around which the magnet wires are wound. In this regard, the lamination stack may be formed with a plurality of slots in which the magnet wires are wound. The magnet wires are coupled at their ends to a commutator, such as to tangs when the commutator is a tang type commutator, disposed on an armature shaft extending coaxially through the lamination stack. The commutator is in contact with one or more brushes, which energize the magnet wires to cause rotation of the armature inside the stator.
In conventional power tools, when the user stops operating the tool by, for example, releasing the tool trigger switch or turning off the power switch, the electric motor is disconnected from the power source and allowed to coast down. Coasting often takes a long time and is undesirable to the user.
As an alternative method to coasting, braking mechanisms have been offers to bring the motor into a halt. One such mechanism is a mechanical brake, which engages the motor shaft and/or tool transmission to stop the rotation of the motor. Alternatively, electronic braking mechanisms may be employed to brake the motor in a controlled fashion. Electronic brake modules often include switching mechanism to short the armature windings and use the current generated by the back electromotive force (EMF) of the motor armature to slow down the armature. This may be done by running current through dedicated brake windings provided in the proximity of the armature windings in the opposite direction of the back EMF current. Alternatively, current from the AC mains may be directed to one or more of the motor field windings in the opposite direction of the back EMF current to slow down the armature.
Conventional electronic brake modules typically utilize multiple mechanical switch or relays for braking. Such circuits tend to be complex and costly. What is needed is a braking circuit arrangement that minimizes switch contacts in order to reduce cost and space.